Alloy



S. UMBREIT MOLYBDENUM TUNGDTEN lllllll ALLOY i I I I I D I I F F FiledNov. 30, 1934 ALLOY INVENTOR. STANTON UMBREIT ATTORNEY.

Patented Oct. 4, 1938 ALLOY Stanton Umbreit, West Orange, N. 1.,assignor, by meane alignments, to Radio Corporation of America, acorporation of Delaware Application November :0, 1934, Serial No.755,313

(oi. sac-21.5)

2 Claims.

My invention relates to alloys of metals having a high melting point,and particularly to alloys of high melting point which are suitable forheatcm, and to cathode heaters for electron discharge I devices.

In the conventional indirectly heated cathode having a heater surroundedby a tubular cathode sleeve the heater wire is'preferably wound into adouble helical coil, and may be mounted on an insulating core, usuallyof alumina. This core and heater wire are sprayed with a suspension ofan insulating material, such as finely powdered alumina, and then bakedat comparatively high temperatures, for example around 1600 C. or

ii higher to harden and consolidate the alumina into a coherent coatingand to fix the heater on the core, producing a cathode heater which maybe mounted within the cathode sleeve.

The ductile tungsten wire often used for cathode heaters is expensive,and is so resilient that the turns of a double helical coil of tungstenwire spring out considerably when the winding mandrel is removed, hencethe wire must be wound on the core with a closer pitch than the finalpitch desired. Because of this resiliency and the fact that it may varywith difierent pieces of wire, the winding pitch may have to be variedfrom piece to piece to get the proper pitch so that the coils are noteasily made to accurate dimensions. The tungsten wire often showsconsiderable brittleness during winding, and regularly shows brittlenessafter the heater has been fired at high temperatures to bake theinsulation on the heaters, which brittleness seems to be due 85 to therecrystallization of the tungsten. Since the tendency in commercialpractice is toward higher and higher firing temperatures to rid thealumina core and insulation of impurities, the brittlenem of thetungsten wire after high tem- 40 perature firing has become a seriousdimculty. It has been found that the heater wire 01' pure tungstenbecomes so extremely brittle after about one thousand hours of life thatsevere jars will often cause the wire to break, opening the cathodeheater circuit and sometimes causing shorts between the cathode heaterand the cathode sleeve. Other common metals have high melting points,such as molybdenum, which has electrical characteristics somewhatsimilar to tungsten.

However, the melting point of molybdenum is so much lower than tungstenthat satisfactory cathode heaters cannot be made of molybdenum wire.

' An object of my invention is to provide an alloy which has forpractical purposes the ad- .66 vantages of tungsten, is ductile andmalleable,

can be easily formed to desired dimensions, and which will not becomebrittle when heated to comparatively high temperatures or during use asa cathode heater.

Another object is to provide an improved cath- 5 ode heater.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims, but theinvention itself will best be understood by refer- 10 ence to thefollowing description taken in connection with the accompanying drawing,which shows a cathode assembly in elevation having a heater coil of wiremade in accordance with my invention and with parts broken away to showdetails of construction.

Referring to the drawing the insulating core Ill, preferably of alumina,carries a heater coil ll of alloy wire made in accordance with myinvention and wound into a double helical coil. 20 The turns of the coilare insulated by a coating of refractory insulation i2, preferablyalumina, which is sprayed on the coil and core in the form of finelypowdered alumina in suspension. The sprayed assembly is fired inhydrogen to a temperature sufiiciently high to bake the insulation onthe coil and core and to drive out the impurities from the insulationand core. The cathode heater is then positioned within the cathodesleeve I! having an emitting coating [4. During operation of an electrondischarge device in which the cathode assembly may be incorporated, heatis conducted and radiated from the heater to the cathode sleeve to raiseit to operating temperature.

In accordance with my invention the heater coil is made of wireconsisting of an alloy of tungsten and molybdenum, in which themolybdenum predominates and for best results the molybdenum shouldcomprise approximately of the entire content of the alloy. While ofmolybdenum and only 10% of tlmgsten have been used, an alloy of thiscomposition has a melting point dangerously close to the temperaturewhich the heater attains when the cathode is aged, that is when it isheated for definite periods of time at high temperature to developemission from the electron emitting coating on the cathode. If the alloycontains too much tungsten, for example over 50%, the objection- 5o ablecharacteristics of tungsten appear, and the working of the-alloy becomesdifilcult. The molybdenum should constitute at least 50% of the alloy.An alloy having 50% tungsten and 50% molybdenum may be desired when avery stifi and strong wire is desired as for example where no core isused, in which case the double helical coil must be self supporting.When no core is used at least 30% tungsten should be used to preventdeformation of the double helical core, especially while being sprayedwith insulating material. I,

The alloy may be made in the conventional manner by mixing powderedmolybdenum and powdered tungsten such as is used in making non-sagtungsten wire, such for example as described in the patent to Pacz1,410,499, by tumbling these powders for from eighteen to twentyfourhours. The mixture is then worked into wire by methods suitable formaking ductile tungsten and described, for example in Coolidge Patent1,082,933. In general it is pressed into molds and sintered at from 1000to 1300 0., preferably in a hydrogen furnace, to provide sufiicientstrength to the molded material so that it can be easily handled. Thismolded bar is then treated in hydrogen with a current hav ing a value ofapproximately -95% of the current necessary to fuse the material. Thisresults in a metallic bar which may be sw-aged and drawn to the desiredshape and diameter, after which it can be wound into a double helicalcoil wire with ease.

An alloy made in accordance with my inver tion is ductile and malleableand when drawn into a wire maintains its shape when once formed into acoil and does not have the excessive resiliency of ductile tungsten. Thedifllculties due to brittleness of ductile tungsten in winding andfiring are eliminated. Although recrystallization of the alloy may takeplace when it is fired with the alumina insulation, brittleness is notintroduced by this firing. The electrical resistance between the heaterand cathode is consistently higher than when the heaters are made ofpure tungsten wire. It is found that in cathodes having heaters made inaccordance with my invention the hum that is sometimes present inindirectly heated cathodes is considerably reduced, and that a heaterwire of molybdenumtungsten alloy made in accordance with my inventiondoes not become brittle after several thousand hours of burning. It hasalso been found that very satisfactory grid wires for transmitting tubescan be made from an alloy made in accordance with my invention.

While I have indicated the preferred embodi ments of my invention ofwhich I am now aware and have also indicated only one specificapplication for which my invention may be employed, it will be apparentthat my invention is by no means limited to the exact forms illustratedor the use indicated, but that many variations may be made in theparticular structure used and the purpose for which it is employedwithout departing from the scope of my invention as set forth in theappended claims.

What I claim as new is 1. An indirectly heated cathode having a cathodesleeve, a heater within said sleeve including a. core of insulatingmaterial, a heater wire of an alloy of tungsten and molybdenum mountedon said core and comprising substantially 80% molybdenum and 20%tungsten said core and wire having a coating of refractory insulatingmaterial.

2. An indirectly heated cathode having a cathode sleeve, a heater withinsaid sleeve including a heater wire of an alloy of tungsten andmolybdenum, said molybdenum comprising substantially 80% of the alloy,and a coating of refractory insulating material on said wire.

STANTON ULIBREIT.

